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Re: Dinosaur Discussion List Dictionary



In a message dated 95-10-24 14:27:18 EDT, rowe@lepomis.psych.upenn.edu
(Mickey Rowe) writes:

>> If a character is found throughout a clade but not in any close
>> outgroup of the clade, we presume that it occurs in the clade
>> because it first appeared in the clade's common ancestor. It is,
>> however, possible that the character was present in the outgroups
>> and was lost/modified in all known members of those outgroups, but
>> this is not considered parsimonious...
>
>Yes, all of that is true.  However, the definition of apomorphy
>(insofar as I'm recollecting correctly) synonymizes "apomorphic" with
>"derived".  What you're describing above (and what you gave as a
>definition) describes how a cladist would determine that a character
>state is apomorphic.  Perhaps you were intending to give a working
>definition similar to what you might find in a physics textbook.  I
>think most biologists would be uncomfortable with your definition,
>though, just as Jeff and I were.  They'd be uncomfortable with it
>because it associates the word with the results of a technique as
>opposed to the underlying reality that the technique is meant to help
>us discover.  I believe that goes against the history of the word's
>usage and hence makes it an incorrect definition.
>
>

Here's what I wrote:

>> APOMORPHY--n. A character state present throughout a clade but
>> not present in any close outgroup of the clade.

This is wrong?

I wanted to keep evolution and phyletics out of the definition as much as
possible. If we use apomorphies to establish phylogenies, then we really
shouldn't use phylogeny to establish apomorphies, yes? So I felt I needed to
be as minimal as possible in the definitions of character, character state,
apomorphy, plesiomorphy, homoplasy, autapomophy, and synapomorphy. The fact
that an apomorphy occurs in a clade specifically because it arose precisely
in the common ancestor is already a conclusion drawn from the distribution of
the character state coupled with the theory of evolution and continuity of
life. Although historically cladistic analysis arose out of the theory of
evolution, in the definitions we may turn this around and bring the theory of
evolution out of cladistic analysis. (Likewise, mathematics is now grounded
in axiomatic set theory--so that set theory "comes first"--but historically,
set theory came much later.)

Of course, we can always add clarifying text and examples to the definition.

G.O.